Primary Distillation of Crude

A typical configuration for the distillation of a crude oil unit includes two main columns, an atmospheric tower and a vacuum tower (see Figure 6). In the atmospheric tower, crude oil is rectified (at a pressure no greater than 275.8 kPa (40 psi); to yield a distillate product containing light hydrocarbon gas, light and heavy naphtha, kerosene, diesel oil, and a bottom product of heavier components (TBP greater than 420°C). Each of the side streams of the atmospheric tower are sent to side strippers that have a partial reboiler or steam stripper. The side stream strippers serve to remove the light components. Stripping by

Figure 6 Crude oil distillation unit.

steam is also frequently used in the bottom of the tower.

The bottom product of the atmospheric tower is further separated by rectification in the vacuum tower. The feed-tray pressure of a vacuum tower is usually 6 kPa (45 Torr). Vacuum towers are mainly designed to obtain heavy distillates such as gas oil, lubricating oils and bunker fuels with asphalt as the bottom product.

The pump-around systems shown in both of the towers serve to make much larger liquid flows on the intermediate stages and produce a net increase in liquid flow. This serves as a point of control to keep the plates from running dry.

Highly developed procedures for the preliminary design of fractionators that process petroleum are commercially available through computer programs. The program 'REFINE' of the ChemShare Corporation and the 'PROCESS' (now PRO-II) program of Simulation Sciences Inc. are two examples.

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